b'YOUR TOOLKIT FOR BUILDING EXCELENCEin most animal confinement buildings and many largenegative the electrical potential of the metal, the greater commercial and industrial buildings. Bird droppings,the oxidation potential. Thus, metals in Table 1 are listed especially around HVAC components, can create healthfrom highest (top) to lowest (bottom) oxidation potential. issues. Birds are notorious for pecking holes throughIn bi-metallic corrosion parlance, the metal with the higher vapor retarders and insulations that are reachable from aoxidation potential becomes the anode, and the other metal roosting location (Figure 5). Such holes generally increasethe cathode as electrons begin to flow between them. The condensation and unwanted air movement, which in turngreater the difference in the oxidation potentials of the two tends to boost decay and corrosion in the surrounding region metals (i.e., the further they are apart in Table 1), the more rapid will be the deterioration of the anode. The relative sizes of the anode and cathode also impact the rate of corrosion. If the metal comprising the cathode is large and surrounds a relatively small anode, the rate of corrosion will be rapid. Conversely if the surface area of the anode is large relative to the cathode, the corrosion rate will be slow.FIGURE 5. BIRD DAMAGE.CorrosionAfter their manufacture, all metals have a propensity to revert back to their more natural and stable state of ore (e.g., iron ore). This naturally occurring, spontaneous and irreversible redox reaction is called corrosion, and the product of this process is generally an oxide, hydroxide or sulfide.Metals such as magnesium, zinc, aluminum and some aluminum alloys have a high oxidation potential and typically undergo corrosion as soon as they are exposed to air. This dry corrosion results in the formation of anoxidelayer/coatingonthemetalsurface.Asthe oxide coating thickens, corrosion slows and eventually stops. If the oxide coating is not physically or chemically disturbed, it will serve as an effective barrier against further corrosion. Since dry corrosion seldom results in a significant loss of base metal and provides some protection against further corrosion, it can be considered beneficial.For most other metals, corrosion reactions typically require an electrolyte (i.e., ionized liquid or gel). Such wet corrosion reactions can significantly deteriorateSEE PAGE 20 FOR FULL CHARTmetal.Bi-metallic corrosion (a.k.a. galvanic corrosion)Preventing wet corrosion in existing buildings is largely is wet corrosion involving two dissimilar metals. Whenabout excluding or removing electrolytes from metals. joined together by an electrolyte, the metal with theIn short, keeping metals dry. This is not an option for higher oxidation potential will corrode. Table 1 lists theHVAC and plumbing systems which involve water and other electrical potential of metals in saltwater where the moresolutions. For an overview of some of the unique corrosion continued on page 16 FRAME BUILDER - JAN2023 / 15'